The present invention relates generally to fluid seals, and more particularly to oil and grease seals having certain constructional features which enable them to prevent generation and build up of high vacuum levels between the so-called primary lip and the auxiliary lip of the seal.
According to customary seal practice, conventional oil seals include a rigid casing portion and an elastomeric seal lip. The primary lip is comprised of a pair of frusto-conical surfaces--"oil" and "air" side surfaces meeting along a seal band to retain lubricant within the sealed region; in common practice, the angle between the shaft being sealed and the oil side surface is a steeper angle or of higher numerical degree than is the angle between the air side lip and th shaft being sealed. This arrangement has customarily provided generally satisfactory sealing in a number of applications, including a certain inherent amount of seal "pumping". However, in recent years, as sealing requirements became more severe, a number of seal designs were provided to achieve further improved or stronger pumping action.
A "pumping" seal is one wherein rotation of the sealed part relative to the seal creates a hydrodynamic action in the sealed fluid at the interface between the seal and the relatively rotatable sealed part. The purpose of this action is primarily to insure against dynamic leakage, and further, to insure that whatever small amount of fluid may escape the sealed region is pumped back by this hydrodynamic action into the sealed area. This takes the form of reducing the height or radius of the meniscus at the primary seal lip and insuring that, while there may be some initial or static leakage, on a net basis, there is substantially zero leakage because leaked fluid is returned to the sealed region by "pumping".
While not every design proposed in the patent literature for constructing a hydrodynamic seal has been effective, a number of such seals have proved effective in use.
Another feature of known oil seals over the years has been the inclusion of a so-called excluder or auxiliary lip. The construction of this lip is such that gross contaminants, such as grit and dust, are kept away from the immediate vicinity of the primary seal so as not to create abrasion or otherwise interfere with the sealing action or effective life of the seal at the primary seal band. Excluder lips have customarily been of several types; one is a lip which, although shorter and stiffer, is somewhat the reverse of a primary lip in the sense of having the surface facing away from the sealed region more steeply angled than the surface facing the inter-lip area. A typical lip of this kind, being spaced apart from the primary lip and exposed on at least one of its surfaces to ambient conditions, operates with little or no lubricant and usually runs at a higher temperature than the primary lip. While the primary lip usually employs a garter spring to secure a tight fit over the shaft, excluder lips rarely, if ever, use such springs.
Other excluder lips are of a thin, flexible cross-section, and still others are intended only to exclude the largest of contaminants and use little, if any, so-called residual or molded interference; that is, the inside diameter (ID) of the seal is usually larger or the same size as the shaft; sometimes the lip is slightly, but only slightly, smaller than the size of the shaft. In the primary lip area, there is a considerable interference or negative difference in sizes so that the seal will closely surround the shaft and needs to be stretched the shaft the same to insure effective sealing.
Whatever their design, most excluder lips solved some problems but created others; if the lips were too loose they were ineffective, and if too tight they created excess heat.
With the advent of improved seal materials, especially filled and unfilled fluorocarbon resins, it was proposed to have auxiliary or excluder lips made from or coated with a resinous fluorocarbon material. The advantage of such polytetrafluoroethylene (TFE) lips is that they run cooler and, being inherently lubricous, require less lubricant than rubber when running over a rotary shaft.
Still further, TFE has good temperature resistance, does not break down chemically and is chemically impervious to oils and their additives. TFE seals have certain drawbacks, however, including being less elastomeric and hence less able to follow the eccentricities in the shaft. While TFE does not itself resist abrasion well, it can function satisfactorily as an excluder lip if the material to be excluded is not entrapped beneath the seal lip.
Referring now to a specific problem with which the invention is concerned, the combined presence of a primary seal lip having a pumping action and a secondary seal with a relatively snug fit over an associated shaft, have served to create an actual vacuum or negative pressure in the region just outside the primary lip. Thus, in the inter-lip area between the auxiliary lip and the primary lip, where the auxiliary lip fits tightly enough over the associated sealed shaft to be air-tight or fluid-tight, the pumping action tending to withdraw oil from the inter-lip region also withdraws air therefrom. As a consequence, with the auxiliary lip not venting towards the inter-lip area, because of its geometry, and the primary lip withdrawing air and oil from this region, a vacuum occurs which is detrimental to seal performance. A high vacuum tends to draw the primary lip increasingly closely over the seal, can tend to unbalance the seal and create primary seal leakage, or can damage the auxiliary or excluder lip. In this connection, drawing the primary lip too closely over the seal causes excess wear, changes the lip geometry, and alters the pumping action.
Previous attempts to deal with this problem have provided excluder lips which were not tight enough to exclude finer contaminants, or accepting shorter seal life or reduced pumping performance of the primary seal lip. Accordingly, while there has been a demand for a seal which provides the requisite pumping action and have good excluder lip characteristics, seals which have achieved at least part of these results have been characterized by occasional, frequent, or regular creation of a damaging vacuum condition in the inter-lip area with the result of short or long term seal failure premature in respect to normally anticipated seal lives.
According to the present invention, a seal is provided which has a relatively conventional pumping type primary lip in combination with a novel TFE or other lubricous flurocarbon auxiliary lip, which is adapted to fit snugly over the associated shaft, but to deflect under vacuum conditions so as to lift slightly away from the sealed shaft and vent the inter-lip area to reduce leakage and to maintain proper lip geometry and force balance in this application.
In view of the failure of the prior art to provide a satisfactory pumping seal having a combination of good excluder action and vacuum venting in the inter-lip seal area, it is an object of the present invention to provide a seal having a hydrodynamic primary lip and a specially designed auxiliary lip adapted to achieve excellent dirt exclusion and to provide venting action in use so as to prevent premature wear and satisfactory performance in use.
Another object of the invention is to provide an oil seal having a resinous fluorocarbon auxiliary lip of a particular design.
A further object of the invention is to provide a seal having a hydrodynamic primary lip and an auxiliary fluorocarbon lip adapted to vent vacuum in the inter-lip area.
A still further object of the invention is to provide an oil seal having a casing and an elastomeric primary lip in combination with a clinched or bonded auxiliary fluorocarbon resin seal lip adapted to run in close sealing engagement with an associated shaft, but to be designed to vent or relieve vacuum under predetermined conditions.
A still further object of the invention is to provide a seal with an auxiliary lip which is curled on its inner margin so as to have its innermost edge lie closer to the primary lip than its central area or outer margin.
Yet another object of the present invention is to provide a fluid seal having a hydrodynamic primary lip and an auxiliary lip which is of completely or partially frusto-conical form in at least one portion so as to provide desirable venting geometry.
A still further object of the invention is to provide a seal with an auxiliary flurocarbon lip which is of frusto-conical shape and is bonded to a portion of the primary seal lip adjacent the area at which the primary lip is bonded to the casing of the seal.
A still further object of the invention is the provision of a fluid seal which may provide a strong pumping action without providing the potential for drawing a measurable vacuum in the inter-lip area.
An even further object of the invention is to provide a method of making seals having the foregoing advantages, characteristics and configurations.
The invention achieves its objects and advantages by providing a seal with a conventional primary lip bonded to a sealing casing and further including, either clinched or bonded thereto, an auxiliary flurocarbon resin excluder lip or ring having its inner diameter adapted for relatively snug sealing engagement with an associated rotary shaft and having the auxiliary lip inner diameter spaced more closely towards the primary lip than its intermediate area or outer margin.
The exact manner in which these and other objects of the invention are achieved in practice will become more clearly apparent when reference is made to the following detailed description of the preferred embodiments of the invention set forth by way of example, and shown in the accompanying drawings wherein like reference numbers indicate corresponding parts throughout.